Kepler's Hunt for Earths Shows Progress at Space Conference

The discovery of exoplanets—worlds around stars other than our own—back in the early 1990s revolutionized the science of astronomy and our perspective on the cosmos. With the launch of NASA's exoplanet-hunting Kepler satellite last year, astronomers think it's less of an "if" but rather a "when" that an earthlike world is spotted around another sun. New research indicates that orange dwarf stars may be the just-right Goldilocks stars for long-term existence of life-supporting worlds, pointing the way for Kepler in its ongoing quest.

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WASHINGTON, D.C.—The search for worlds like Earth around other stars marches on at the American Astronomical Society (AAS) winter conference this week. Scientists for NASA's exoplanet-hunting Kepler mission report that the orbiting space telescope has nabbed five new worlds, the first several of many that astronomers hope it will eventually discover. Meanwhile, other researchers have determined that stars a bit cooler and smaller than our sun, called orange dwarfs per their color, may be the best kind of stars for hosting planets with advanced, intelligent civilizations.

Kepler's mission is to spy on stars likely to possess earthlike planets, so many of these orange dwarfs already reside in the satellite's field of view. Earthlike exoplanets around these orange dwarfs will be easier for Kepler to detect than those around larger, more luminous stars like our sun, says Edward Guinan, an astronomer and astrophysicist at Villanova University. Guinan, along with undergraduate Aaron Wolfe, presented the orange dwarf research in an AAS poster session on Jan. 5.

The Kepler results were announced on Jan. 4, and will be published Thursday in Science. "It's very exciting that we've found these new planets, and it shows us that the instrument is working very well," says William Borucki of NASA's Ames Research Center and the principal science investigator for Kepler, which won a Popular Mechanics Breakthrough Award in 2009.

The five exoplanets that appeared in Kepler's crosshairs range in size from Neptune to even bigger than Jupiter. All orbit close to their stars and with temperatures over 2000 degrees Fahrenheit, making these worlds hotter than magma and summarily unfit for life. Nevertheless, the findings will help scientists further understand the physics and characteristics possible for exoplanets. One of these newfound exoplanets is already straining theory due to its mysteriously low density—on par with that of Styrofoam, says Borucki. "This is one of the lightest planets ever found," he says.

Stars that are Just Right

Kepler, which launched in March 2009, spots exoplanets as they pass in front of their stars. This "transit" causes a slight dimming in the star's brightness. For an earthlike world—tiny on the planetary scale—this brightness dip is only about one part in 10,000, making the detection of terrestrial bodies difficult. To compensate, Kepler is monitoring 150,000 stars simultaneously for these slight eclipses.

For worlds found in the habitable, or Goldilocks, zone—where water neither boils away nor freezes permanently—these transits would occur only about once every Earth year around a star like our sun. Therefore, it will take several years for Kepler to detect and then re-detect most terrestrial candidates around sunlike stars, which will then be subjected to further verification using ground-based telescopes. If earthlike planets are indeed commonplace, as scientists believe, Kepler should spot hundreds of such worlds by the end of 2012.

Small stars that are cooler than our sun and more copperish in complexion look like good candidates for hosting this expected windfall of earthlike havens, and stand as the most promising in terms of being home to intelligent alien life, researchers say. Technically called main-sequence K dwarfs, or orange dwarfs, these stars are about 60 to 85 percent of the mass of our sun and less than half as bright.

Given these qualities, the stars may in fact be optimized for supporting life on their habitable-zone planets; they're Goldilocks stars for Goldilocks planets. That's because stars cooler and smaller than K-class stars, designated M-class or red dwarfs, cannot support a large or particularly friendly habitable zone. The exoplanets that exist in this narrow band must form so close to their star that they receive large doses of radiation, a situation not conducive to developing life. Plus, exoplanets around red dwarfs can become tidally locked to their star, Villanova's Wolfe says, meaning that one side of the planet constantly faces the star, roasting under perpetual sunshine, while the other half of the planet is plunged into an endless, cold night.

Warmer than K-class stars are G-class stars like ours, which obviously can support life, Guinan says. G-class stars last approximately 10 billion years before they balloon into the bloated red-giant phase that many stars pass through. When our sun goes red giant in about five billion years, it may swallow up Earth entirely. Humankind, at least earthbound, will likely be toast by then anyway, says Guinan, as our sun will heat up significantly over the next billion years.

Orange dwarfs, on the other hand, shine for 20 to 50 billion years. The much-longer window of biological sustainability on their exoplanets gives life plenty of time to get going and evolutionarily advance, Guinan says. Life on Earth began about 3.8 billion years ago, meaning we'll end up with a biologically friendly era totaling about five billion years. "These dwarf K stars, however, would provide their planets in the habitable zone with a stable climate for perhaps 20 billion years," Guinan says.

For these reasons, Guinan continues, "we think it's reasonable that advanced, intelligent civilizations would most likely be found around these [K-class] stars." He says an increasing number of exoplanets have been found around orange dwarfs, which are perhaps 10 times as numerous as stars like ours.

Over 400 exoplanets have been discovered to date, the vast majority of which are "hot Jupiters." Fortunately, Guinan says, a large number of K-type dwarf stars are in Kepler's field of view. Because these K-class stars are smaller than the sun, their habitable planets would likely orbit in less than an Earth year, meaning that Kepler could spot multiple transits sooner than with G-class stars, leading to speedier identifications.

Thanks to Kepler, an answer to the "are there other Earths?" question may be answered in the affirmative tantalizingly soon—maybe even at another AAS meeting in coming years.

"The Kepler mission is about whether Earths are common in our galaxy," NASA's Borucki says. "If Earths are common, then there probably is a lot of life in our galaxy. If not, then we are alone."